Paper-based microfluidic analytical devices are attractive for use in settings where conventional laboratory diagnostics are unsuitable or undesirable, for example, in developing regions, remote regions, emergency situations, and home healthcare. Paper-based devices comprise paper, wax and assay reagents that are pre-deposited onto the paper. Typically, hydrophobic regions patterned in the paper substrate may define isolated hydrophilic zones of the paper substrate for conducting, for example, biological assays, or hydrophilic channels that may direct the movement of fluid to an assay zone.
Known methods for forming such regions include printing, for example, via jetting, of wax-based ink onto the surface of a paper substrate, followed by heating of the substrate to melt (reflow) the wax through the thickness of the paper, leading to the formation of hydrophobic barriers that define hydrophilic regions of paper substrate. Because the conventional, wax-based inks are designed to stay on top of paper after being jetted, the heating step is necessary so that the wax reflows to penetrate the thickness of the paper to create the isolated hydrophilic zones.
As depicted in FIG. 11A and FIG. 11B, one limitation of such a method is that the conventional wax ink must be melted 9 refloxed after it is deposited on the substrate in order to penetrate into the substrate, and such melted wax ink spreads isotropically through the paper.